5

Conservation Principles

As simple as possible, but no simpler.”

—Albert Einstein, 1879–1955, Physicist

I.    THE SYSTEM

As discussed in Chapter 1, the basic principles that apply to the analysis and solution of flow problems include the conservation of mass, energy, and momentum, in addition to appropriate transport relations for these conserved quantities. For flow problems, these conservation laws are applied to a system, which is defined as any clearly specified region or volume of fluid with either macroscopic or microscopic dimensions (this is also sometimes referred to as a “control volume”), as illustrated in Figure 5.1. The material (fluid) within the system is assumed to be a continuum, with physical properties that can be defined ...

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